The heart is the core muscle responsible for pumping life-sustaining blood through the body via an intricate network of vessels. It works ceaselessly and beats 100,000 times a day and 40 million times a year.
In its simplest form, the human heart can be described as a four-chamber structure, each chamber filling with a new round of blood with every beat. The chambers are called the right atrium, left atrium, right ventricle and left ventricle. Each chamber is connected with a valve. These valves operate similarly to check valves and ensure blood flows in the proper direction through the heart. The right chambers receive blood that is low in oxygen and then pump the blood through the pulmonary artery and into the lungs. The left side of the heart receives the now oxygen-rich blood from the lungs and the left ventricle pumps the blood out to the body, through the aorta.
When the valves of the human heart cease to work properly, leakage can occur between the chambers of the heart, resulting in a lower blood pressure or high resistance for the blood to pass through. One solution is to replace malfunctioning valves with either a mechanical or a biological valve.
A stented tissue heart valve is a replacement prosthetic heart valve composed of a stent covered in biological tissue. The stented tissue heart valve replaces the diseased, damaged or malfunctioning valve, such as the aortic valve between the left ventricle and the aorta. It is designed for supra-annular placement. Its functional purpose is to maintain structural integrity with a high-strength fatigue-resistant stent at the core.
Through cardiac surgery, the malfunctioning heart valve can be removed and then replaced by the stented tissue valve. Selection of the appropriate size replacement valve is of great importance. Prosthetic heart valves typically have a diameter between about 19 mm and about 29 mm. The valve size selection is determined through a sizer-replica of the 19-29 mm diameter valve. Once an appropriate size is selected to fit a patient, sutures are sewn into the aortic tissue. The sewing cuff of the prosthetic valve is then threaded over these sutures, and the valve is transferred down to the aortic opening where it is firmly attached.
Numerous replacement prosthetic heart valves have been designed. Conventional prosthetic heart valves are manufactured from a metallic stent assembly and bovine pericardium tissue. The stent assembly consists of a core (called a stent) formed from metal, such as titanium alloy, a polyester fabric forming the sewing cuff and porcine tissue covering all edges of the stent. The bovine pericardium tissue is attached to the stent assembly to form three leaflets which cooperate to permit blood to flow in one direction, but not the other.
An example of a stent 10 for use in a conventional prosthetic heart valve is shown in FIG. 1, and is discussed more fully in U.S. Patent Application Publication No. 2008/0147179, the disclosure of which is hereby incorporated herein by reference. Stent 10 is an annular structure formed from metal, such as titanium. In a typical process, stent 10 may be formed by laser cutting a titanium tube to the desired shape, followed by electro-polishing the resultant structure. The stent structure includes a base 4, commissure posts 60A-60C extending from the base, a blood inflow edge 12, and a blood outflow edge 14. Stent 10 can have a diameter ranging from about 19 mm to about 29 mm and a wall thickness PAT ranging from about 0.25 mm to about 0.33 mm, depending on the selected size of the stent. The width W of each commissure post typically ranges from about 1.45 mm to about 1.51 mm.
Numerous geometrically-shaped openings are provided within metallic stent 10. Elongated openings 22A, 22B, and 22C extend along the lengths of posts 60A-60C. The portions of elongated openings 22 closer to inflow edge 12 are generally rectangular in shape, whereas the portions of elongated openings 22 closer to outflow edge 14 are more triangular in shape. Each elongated opening 22 includes five distinct edges 24A,24B,24C,24D, and 24E that generally form the shape of an elongated bottle.
Openings 18A-18F extend around the circumference of base 4. Each opening 18 includes three distinct edges 20A, 20B, and 20C that generally form the shape of a right triangle, and more specifically a 30-60-90 right triangle. Edge 20C is directly adjacent an edge 24A or 24D of an opening 22 in a commissure post. As shown, openings 18, 22 are positioned a predetermined distance away from the inflow and outflow edges of stent 10. As a result, inflow edge portion 28 has a width PA1 between inflow edge 12 and edges 20A of openings 18, and a width PA3 between inflow edge 12 and edges 24E of openings 22. Similarly, outflow edge portion 30 has a width PA2 between outflow edge 14 and edges 20B of openings 18, and a width PA4 between outflow edge 14 and edges 24B, 24C of openings 22. The widths PA1 and PA3 of inflow edge portion 28 are substantially similar to the widths PA2 and PA4 of the outflow edge portion 30. Such dimensional uniformity in stent 10 is believed to provide a stable structure that can minimize deformation of the stent, especially during handling of the stent by surgeons.
Despite the improved design of the stent shown in FIG. 1, there is still room for further improvements. For example, because of the plastic behavior of metals, metallic stents are subject to deformation during handling and implantation in a patient. A need therefore exists for prosthetic valves having improved stent designs that are less prone to deformation, but that are capable of reliable production.